My Wheaten Terrier keeps chewing on drywall. Is it possible he's lacking something in his diet or just a bad habit.

Reading other peoples responses, he may be bored. Although, he does have his sister here and they play well together. He has an embarrassing amount of toys and a dog door to go outside during the day.
When I show the chewed areas to him, he lowers his head and feels guilty. I have 1/2 the kitchen walls covered with dog gates where he has chewed and he just keeps moving down the line. I guess I'll try the bitter apple?

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If you haven't tried those large Leather Chew Toys, that may be of some help.

But, if it's a Dietary issue...
There are some differences in Breeds for Dietary intake, you might do a search for your specific Breed, and compare that with the Food you are feeding presently. Adjust/supplement/change as necessary.

I ate drywall as a pup and a desperate Mom called the vet. She thought it was a teething issue at the time. The vet said I had a calcium deficiency and put me on a suppliment. After a year old mom stopped the tabs and again I began eating drywall. Mom called the vet and he said to wean me off of the calcium. It worked and I no longer crave the stuff. Consult with your vet. Good luck!

boredom. he is not getting enough exercise. having a sister to play with is not enough mental stimulation for any dog or any terrier. Terriers have to have a resource to blow off excess energy.
I took my dog running in the woods and we both felt so excellerated inside. It was such good healthy stimulation to run in the woods like that. We both I am sure felt energized and free :) of course, be safe and keep your dog leashed at all times.
you might want to consider, jogging, running, ball games , running in the park, frisbee, or biking with your dog instead of bitter apple spray. Motto::: A tired dog is a happy dog :)